[PATCH v2] arm64/gcs: Fix error handling in arch_set_shadow_stack_status()

[Breno Leitao <leitao@debian.org>]

alloc_gcs() returns an error-encoded pointer on failure, which comes
from do_mmap(), not NULL.

The current NULL check fails to detect errors, which could lead to using
an invalid GCS address.

Use IS_ERR_VALUE() to properly detect errors, consistent with the
check in gcs_alloc_thread_stack().

Fixes: b57180c75c7eb ("arm64/gcs: Implement shadow stack prctl() interface")
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
---
Changes in v2:
- Remove unnecessary PTR_ERR() conversion (Will)
- Link to v1: https://patch.msgid.link/20260130-arm64_cgs-v1-1-c9b6942b6b70@debian.org
---
 arch/arm64/mm/gcs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c
index 6e93f78de79b1..04a23a497f205 100644
--- a/arch/arm64/mm/gcs.c
+++ b/arch/arm64/mm/gcs.c
@@ -199,8 +199,8 @@ int arch_set_shadow_stack_status(struct task_struct *task, unsigned long arg)
 
 		size = gcs_size(0);
 		gcs = alloc_gcs(0, size);
-		if (!gcs)
-			return -ENOMEM;
+		if (IS_ERR_VALUE(gcs))
+			return gcs;
 
 		task->thread.gcspr_el0 = gcs + size - sizeof(u64);
 		task->thread.gcs_base = gcs;

---
base-commit: 8dfce8991b95d8625d0a1d2896e42f93b9d7f68d
change-id: 20260129-arm64_cgs-496817025d1e

Best regards,
--  
Breno Leitao <leitao@debian.org>